The present invention is related to a method of forming a dielectric of the type used in semiconductor applications, and more particularly to a process for improving selectivity and hermeticity characteristics of etch stop films and hard mask layers using UV treatment.
As integrated circuits (IC) feature sizes shrink, problems of increased resistance and resistance-capacitance (RC) coupling offset any speed advantage derived from the smaller device size, limiting improvement in device performance. Ways to improve device performance and reliability include using highly conductive metals, such as copper, and employing lower dielectric constant (low-k) materials.
Low-k materials are, by definition, those semiconductor-grade insulating materials that have a dielectric constant (“k”) lower than that of SiO2, i.e., 3.9. Various types of low-k materials can have dielectric constants ranging for instance from about 3.5-3.9 (e.g., FSG), to less than about 3.2 (e.g., CDO, “Carbon Doped Oxide”), to as low as 2.0-2.2 (e.g., films with intentionally introduced porosity, such as porous MSQ) or even lower.
An etch stop film is used in various damascene processes to retard etching. Having an etch stop film that etches slower than the dielectric layer above or below defines a uniform depth for metal lines and sometimes allows simultaneous etching of trenches and vias. The etch stop film is usually deposited between two layers of dielectrics, although other configurations exist.
A good etch stop film would be highly selective, or be more resistant to etching, over the other films etched at the same time. Conversely, a thicker film of a less selective etch stop film would be needed to protect the dielectric layer because it would be etched faster than a highly selective film under the same conditions. A thinner etch stop film is desirable because a thicker etch stop film increases the overall k value, decreasing the RC benefits of using low-k dielectrics. Further, thinner films use less material and machine time, decreasing manufacturing costs.
A good etch stop film would also be hermetic to penetration by various compounds intentionally or unintentionally present during semiconductor manufacture, including for instance moisture and amines. Moisture is ubiquitous during semiconductor manufacture, but moisture penetration in a dielectric layer is undesirable. Penetration by moisture is one of the hardest things to prevent, since the water molecule diffuses rapidly through many dielectrics. Hermeticity is therefore often used as a measure of the moisture barrier capability of a deposited film and can be measured relatively. In general, moisture penetration in dielectric films causes an increase in dielectric constant that degrades device performance. It also causes reliability problems such as reduction in electromigration lifetime, enhanced stress migration and reduced Time Dependent Dielectric Breakdown (TDDB) for a number of reasons among which are poor adhesion, increased line-to-line leakage, metal oxidation and corrosion of the copper lines. In the case of a highly doped fluorosilicate glass (FSG), a commonly used dielectric, moisture cleaves the Si—F bond liberating mobile fluorine. The presence of mobile fluorine and hydrogen leads to the eventual formation of HF, which causes adhesion problems and subsequent blistering of the dielectric films. The moisture will also change the stress characteristics of the dielectric film, causing unwanted stress drift. Some etch-stop layers also serve as dielectric diffusion barrier layers that are deposited on the copper lines and are used to prevent interlayer copper diffusion. For this application, it is critical that the layer be hermetic for reliability.
A hard mask is used in a fashion similar to the etch stop film. A hardmask is typically used in dielectric etch when the material to be etched has a similar etch rate as the photoresist used to pattern the layer. In this case, a hard mask would be patterned with an etch chemistry that does not substantially etch the photoresist or underlying layer. The underlying layer can then be etched with a chemistry that does not etch the hardmask significantly. An example would be a nitride hardmask over an organic low k layer.
A hard mask can often be used as a CMP stopping layer as well. After removing excess metal from the top of the dielectrics it is sometimes desirable to have a film present that is not easily removed or damaged by CMP to for instance preserve softer low k layers underneath.
What is needed, therefore, is a method of making etch stop films and hard masks that have high selectivity and are more hermetic to improve device performance and reliability.